1. Field of the Invention
This invention relates to a power factor correction converter capable of fast adjusting load.
2. Description of the Prior Art
With reference to FIG. 1 shown as a schematic view illustrating a conventional single-phase rectification circuit, a bridge rectification circuit 10 consists of four diodes 11, 12, 13, and 14 that are connected in parallel to an output capacitor C1. An AC power P1 is further connected across the bridge rectification circuit 10. When the AC power P1 is positive, the input current is transmitted through diodes 11 and 13; when the AC power P1 is negative, the input current is transmitted through diodes 12 and 14 and then filtered by the output capacitor C1, a DC power being thereby obtained. Although the structure of rectification circuit is advantageously simplified, the bridge rectification circuit 10 that charges the output capacitor C1 easily causes a very high surge current impacting other sets of electrical equipment.
With cross reference to FIG. 2 shown as a view of an output voltage waveform generated from the single-phase rectification circuit in FIG. 1, when the input voltage waveform W1 generated from the AC power is a sine wave and the output power waveform W2 contains DC power, the surge current W3 generated from the input bridge rectification circuit 10 is not a sine wave; in addition to inferior power factor, the surge current makes the capacity of components and wiring circuit increase, the loss of power supply system thereby directly increasing and other users applying a power distribution system being thereby indirectly affected.
Owing to the poor effects derived from the conventional manners, in many prior arts, the technology of power factor amendment is used to improve the art. With different components, the technology of power factor amendment may be divided into passive and active power factor correction. The circuit of passive power factor correction is easily designed, in which a filtering circuit consists of an inductor and a capacitor (not shown) is added between the bridge rectification circuit 10 and an AC power P1 to moderate the surge current for enhancement of the power factor. However, in such a manner, the total harmonic distortion of input current is high, the physical volume is extremely high, and the power factor is not effectively improved.
In the aspect of active power factor correction, the circuit is more complicatedly designed and a switch component must be added in the circuit; further, in the electrical and electronic technology, an adequate control manner is applied to turn ON or OFF the active power switch, and thus the input power current is made to approach the sine wave and follow the input power voltage; the power factor may reach 0.97 or above and there are advantages of low physical volume, low weight, and low total current harmonic distortion.
With reference to FIG. 3 shown as a schematic view illustrating a conventional control circuit provided with a single-phase active PFC specific IC (UCC3854), a potential-divider resistor 20 is mainly used to obtain a DC output voltage feedback signal and, after the signal is compared by a voltage amplifier 21 with a DC voltage command Vref, a voltage differential signal A is obtained; then after the signal is rectified by the diode-based bridge rectifier 22, an input voltage signal B is obtained by a resistor 23 and multiplied through a multiplexer 24. Thus, a sine current command co-phase with the input voltage, the amplitude of which is adjusted according to the variation of a load may be obtained. After square times of signal C transmitted through a low-pass filter 25 to the multiplexer 24 is obtained, the signal C is divided by the product of voltage differential signal A and the input voltage signal B; in such a manner, the gain of loop formed by the voltage amplifier 21 may be kept constant and the output is made to serve as a power control. Thus, the variation of input power 26 that is allowed by the system may increase. Next, the sine current command is compared with the feedback of a real input current in a current amplifier 28 and then a current differential compensation signal may be obtained. Further, the current differential compensation signal is compared with a sawtooth wave or a triangle wave Vs in a Pulse Width Modulation (PWM) and then a pulse modulation signal is obtained; next, the signal is converted by a Gate Driver 30 into a drive signal for the active power switch 31 to control the amplitude of duty cycle of the power switch 31. When the real input current is higher than the sine current command, a negative value or a lower current differential compensation signal is obtained from the current amplifier 28 to reduce the duty cycle; otherwise, the duty cycle increases. Thus, the input current may follow the sine current command to vary for making the phase of current of the input power 26 corresponds to that of voltage of the input power 26 and thus increasing the power factor. However, there are many defects in the conventional correction circuit, such as what is described below.                (A) Being implemented with hardware, the structural control circuit is easily limited to the characteristics of all circuit components, and errors caused in a manufacturing process, so it is not easy to implement the control strategy.        (B) With reference to FIG. 4, because the sine current command is obtained according to the input voltage signal B detected by the resistor 23, when the voltage of input power 26 forms a non-pure sine wave, if the current command W4 is applied in this case, harmonic content is contained; thus, the current of real input power 26 and the identical harmonic content of input voltage cause the power factor to be impacted and generate high frequency harmonics of current of the input power 26 that turn worse the quality of power of the power supply system.        (C) Owing to the non-linear characteristic of rectification filtering, the conventional active power factor correction converter causes second harmonics of the power frequency of DC output voltage. In order to reduce the impact of second harmonics, a first-order RC low-pass filter the frequency of which ranges from 10 Hz to 20 Hz is generally added in the path of voltage feedback. Although, in this manner, the second harmonics in the feedback loop may be attenuated to keep stable the DC output voltage, the bandwidth is thus limited, the dynamic response of system being thereby poor. Consequently, when fast DC output voltage connection varies the load, the output voltage cannot be stable. The maximum output voltage overshoot and dip that are caused by the load variation significantly increases to indirectly turn worse the effect of improvement of the power factor and input current harmonics.        
With cross-reference to US patent No. 2006245219, titled Digital Implementation of Power Factor Correction, a digital circuit is provided to implement a conventional active power factor correction converter, and it is disclosed that the feedback voltage is fed forward to a current loop command input terminal to enhance the dynamic response of output voltage.
With cross-reference to Taiwan Laid-Open patent No. 200423516, titled Power Supply Controller for a drive motor of a sports apparatus, a digital processor is used to implement a conventional active power factor correction converter, and it is closed that an input voltage waveform is read in the manner of table lookup to be a basis of modulation of an input current waveform.
Consequently, because of the technical defects of described above, the applicant keeps on carving unflaggingly through wholehearted experience and research to develop the present invention, which can effectively improve the defects described above.